The present invention relates generally to fluorescent lamps that are outfitted so as to provide an increased or more robust light output, particularly when the lamps are in service in a cold environment.
Since those having ordinary skill in the art are familiar with the manner in which fluorescent lamps operate, the details thereof are not set forth here. However, it is to be noted that the interiors of fluorescent lamps comprise vacuums and the lamps typically employ mercury in its gaseous state as one of the elements for producing light. When a fluorescent lamp is turned on, an electric arc is established across the length of the lamp between the electrodes or cathode stems at opposite ends of the lamp and liquid mercury within the lamp is vaporized. The gaseous mercury, after colliding with electrons and charged atoms within the lamp, releases light photons in the ultraviolet wavelength range. These light photons interact with phosphors in the lamp to produce white light. The colder the interior of the lamp, the lower will be the vapor pressure of the mercury so that more heat will be required to vaporize the liquid mercury and cause the vaporized mercury to move within the interior of the lamp with a desired rapidity. Consequently, the output of light from the lamp will be less at lower ambient temperatures.
The vapor pressure of the mercury is governed by the coldest spot in the lamp. Typically, in a fluorescent lamp comprised of an elongated glass tube having cathode stems of substantially equal length mounted at either end of the tube, such as a T8 or T12 fluorescent lamp for example, the coldest site in the lamp is a spot, herein referred to as the “cold spot”, that is located substantially at the mid-point between the ends of the tube. On the other hand, where the internal diameter of the glass tube is sufficiently small and one of the cathode stems is longer than the other and penetrates the interior of the glass tube a greater distance than the shorter cathode stem, as is the case with certain fluorescent lamps such as the T5 fluorescent lamps for example, the coldest location in the lamp is a site nearer the end of the glass tube where the longer cathode stem is in place.
The effect of the cold spot of the fluorescent lamp on the vapor pressure of the mercury and the consequent effect on the light output from the lamp is particularly significant when the lamp is in service in a cold environment such as a refrigerator or freezer. However, even at warmer ambient temperatures, the influence of the cold spot is seen.